1. Field of the Invention
This invention relates to a uniformly doped n-type polyacetylene which has oxidation stability and to a process for producing the same.
2. Description of the Prior Art
Electrically conductive polymers have recently been discovered which make it possible to produce inexpensive products having large surface areas. The flexibility of the polymers render the same useful for making solar cells, diodes, transistors and other elements of semiconductor devices. Polyacetylene, among other electrically conductive polymers, has drawn particularly attention because it can be produced inexpensively and because it provides an excellent element for a semiconductor device.
When an electrically conductive polymer is used in the manufacture of a semiconductor device, it is necessary to control its semiconductor characteristics so that it may be possible to obtain a p- or n-type semiconductor having a large area, heat and oxidation stability and low specific resistance. A p-type polymer semiconductor is easy to produce, for example, by doping polyacetylene with AsF.sub.5, I.sub.2 or HCl. Virtually no n-type polymer semiconductor, on the other hand, has been produced successfully. There has therefore been a need for stable n-type polymer semiconductors.
Various methods have been proposed for synthesizing n-type polyacetylene, but none of these methods have been found capable of synthesizing a stable product. One conventional method of doping polyacetylene is electrochemical doping in which polyacetylene is used as a cathode in a dopant solution, while a platinum or other metal plate is used as the anode in the electrochemical cell. Dopants which have been used in the past include ions of compounds of the following formulas: ##STR2## wherein X is O or N, and R and R' are each an alkyl group (Japanese Laid-Open Patent Publication No. 38743/1983); ##STR3## wherein R.sup.3 to R.sup.6 are each an alkyl group (Japanese Laid-Open Patent Publication No. 38745/1983); EQU H.sub.2 F.sup.+
(Japanese Laid-Open Patent Publication No. 89640/1983); and ##STR4## wherein R.sup.7 to R.sup.9 are each an alkyl group (Japanese Laid-Open Patent Publication No. 93748/1983).
The electrochemical method has the disadvantage of requiring a large apparatus including a source of constant electric current supply and a platinum or other metal plate which is used as the electrode of opposite polarity to the polyacetylene electrode. The use of polyacetylene itself as the cathode has the disadvantage that in the doping process the density of doping ions on the surface of the cathode differs from the density of the interior of polyacetylene, which results in non-uniformity of doping. The n-type polyacetylene prepared by this method is difficult to use in semiconductor devices.
A method has also been proposed which does not rely on electrochemistry, but instead simply involves the dipping of polyacetylene in a dopant solution. In one example, polyacetylene is dipped into a tetrahydrofuran solution of sodium naphthalide. The preparation of a stable dopant is, however, difficult, and the n-type polyacetylene prepared by this prior art method is so unstable that it is readily decomposes upon exposure to the air. It is not suitable for any partical use. A need therefore, continues to exist for a satisfactory method by which n-doped polyacetylene can be obtained.